Terahertz electromagnetic wave generator, terahertz spectrometer and method of generating terahertz electromagnetic wave

ABSTRACT

A terahertz electromagnetic wave generator according to the present disclosure includes: a substrate; a thermoelectric material layer which is supported by the substrate and which has a surface; and a pulsed laser light source system which locally heats the thermoelectric material layer with an edge of the surface of the thermoelectric material layer irradiated with pulsed light, thereby generating a terahertz electromagnetic wave from the thermoelectric material layer.

This is a continuation of International Application No.PCT/JP2014/000927, with an international filing date of Feb. 21, 2014,which claims priority of Japanese Patent Application No. 2013-054779,filed on Mar. 18, 2013, the contents of which are hereby incorporated byreference.

BACKGROUND

1. Technical Field

The present application relates to a terahertz electromagnetic wavegenerator, a terahertz spectrometer, and a method of generating aterahertz electromagnetic wave.

2. Description of the Related Art

In this specification, the “terahertz electromagnetic wave” will referherein to an electromagnetic wave, of which the frequency falls withinthe range of 0.1 THz to 100 THz. 1 THz (terahertz) is 1×10¹² (=thetwelfth power of 10) Hz. Terahertz electromagnetic waves are now used invarious fields including security, medical treatments, andnondestructive tests on electronic parts. Since there are excitation,vibration and rotation modes of various electronic materials, organicmolecules and gas molecules in the terahertz electromagnetic wavefrequency range, people have proposed that a terahertz electromagneticwave be used as a sort of “fingerprint” to recognize a given material.On top of that, since a terahertz electromagnetic wave is safer than anX ray or any of various other electromagnetic waves, the terahertzelectromagnetic wave can be used to make a medical diagnosis withoutdoing harm on the body of a human subject.

As disclosed in Nature Mater. 1, 26, (2002), a photoconductor or anonlinear optical crystal is used as a conventional terahertzelectromagnetic wave generator. In any of those elements, a terahertzelectromagnetic wave is generated by irradiating the element with alaser beam, of which the pulse width falls within the range of a fewfemtoseconds to several hundred femtoseconds (and which will behereinafter referred to as a “femtosecond laser beam”). 1 femtosecond is1×10⁻¹⁵ (=the minus fifteenth power of 10) seconds. In a vacuum, anelectromagnetic wave travels approximately 300 nm in one femtosecond.

Such a terahertz electromagnetic wave is generated by taking advantageof a so-called “dipole radiation” phenomenon in classicalelectromagnetism. That is to say, a variation in electric polarizationor current in accelerated motion with time generates an electromagneticwave at a frequency corresponding to the rate of that variation. Since avariation in polarization or current is induced in a few femtoseconds toseveral hundred femtoseconds (which depends on the pulse width of thelaser beam) by being irradiated with a femtosecond laser beam, theelectromagnetic wave generated by dipole radiation has a frequencyfalling within the terahertz range.

SUMMARY

According to a method of generating a terahertz electromagnetic waveusing a photoconductor, a bias voltage needs to be applied to thephotoconductor. That is why to generate a terahertz electromagneticwave, not only a femtosecond laser diode but also an external voltagesupply are needed. However, to use the terahertz electromagnetic wavetechnologies in a broader range of fields in practice, a method ofgenerating a terahertz electromagnetic wave without using such anexternal voltage supply should be provided so that the technologies workin various operating environments.

According to a method of generating a terahertz electromagnetic waveusing a nonlinear optical crystal, on the other hand, no externalvoltage supply is needed. However, since the second-order nonlinearoptical effect is used, a femtosecond laser beam needs to be radiatedprecisely toward a predetermined crystal orientation of a nonlinearoptical crystal. In addition, the phase matching condition needs to besatisfied, and therefore, the nonlinear optical crystal should bedesigned, shaped and controlled precisely enough.

The present disclosure provides a technique for generating a terahertzelectromagnetic wave using a simpler configuration.

In one general aspect, a terahertz electromagnetic wave generatordisclosed herein includes: a substrate; a thermoelectric material layerwhich is supported by the substrate and which has a surface; and apulsed laser light source system which locally heats the thermoelectricmaterial layer with an edge of the surface of the thermoelectricmaterial layer irradiated with pulsed light, thereby generating aterahertz electromagnetic wave from the thermoelectric material layer.

In another aspect, a terahertz spectrometer disclosed herein includes:the terahertz electromagnetic wave generator described above; an opticalsystem which irradiates an object with a terahertz electromagnetic wavethat is generated by the terahertz electromagnetic wave generator; and adetector which detects the terahertz electromagnetic wave that istransmitted through, or reflected from, the object.

In another aspect, a method of generating a terahertz electromagneticwave disclosed herein includes: (A) providing a thermoelectric materialbody; and (B) locally heating the thermoelectric material body byirradiating a portion of the thermoelectric material body with pulsedlight. The step (B) includes: locally heating the thermoelectricmaterial body so that an asymmetric heat distribution is formed in thethermoelectric material body; and producing thermal diffusion current inthe portion of the thermoelectric material body that is heated locally,thereby generating a terahertz electromagnetic wave.

According to the present disclosure, by irradiating a thermoelectricmaterial layer with a femtosecond laser beam, a macroscopic current canbe induced, and a terahertz electromagnetic wave can be generated fromthis current.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates how the Seebeck effect is produced.

FIG. 1B is a perspective view illustrating a terahertz electromagneticwave generator according to the present disclosure.

FIG. 1C is a schematic representation illustrating a terahertzelectromagnetic wave generator according to the present disclosure.

FIG. 2A is a cross-sectional view schematically illustrating how theterahertz electromagnetic wave generator 4 operates when a femtosecondlaser beam 5 is incident on the generator 4.

FIG. 2B is a top view schematically illustrating how the terahertzelectromagnetic wave generator 4 operates when a femtosecond laser beam5 is incident on the generator 4.

FIG. 2C is a top view schematically illustrating how the terahertzelectromagnetic wave generator 4 operates when a center portion of thesurface 20 of the thermoelectric material layer 2 is irradiated with thefemtosecond laser beam 5.

FIG. 2D illustrates an exemplary configuration for a terahertzspectrometer according to an embodiment of the present disclosure.

FIG. 2E is a flowchart showing the procedure of generating a terahertzelectromagnetic wave.

FIG. 2F is a cross-sectional view schematically illustrating a terahertzelectromagnetic wave generator 4 with a thermoelectric material layer 2which has been patterned into a linear shape.

FIG. 2G is a top view schematically illustrating a terahertzelectromagnetic wave generator 4 with a thermoelectric material layer 2which has been patterned into a linear shape.

FIG. 2H is a cross-sectional view schematically illustrating a terahertzelectromagnetic wave generator 4 including a thermoelectric materiallayer 2 with a hole.

FIG. 2I is a top view schematically illustrating a terahertzelectromagnetic wave generator 4 including a thermoelectric materiallayer 2 with a hole.

FIG. 3A is a perspective view illustrating a situation where a terahertzelectromagnetic wave generator comprised of Bi and SiO₂ is arranged sothat a right half of the area irradiated with a laser beam will be Biand a left half of that area will be SiO₂.

FIG. 3B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and SiO₂ was irradiatedwith a femtosecond laser beam and was arranged so that a right half ofthe area irradiated with the laser beam would be Bi and a left half ofthat area would be SiO₂.

FIG. 4 is a graph showing the power spectrum of the terahertzelectromagnetic wave shown in FIG. 3B.

FIG. 5A is a perspective view illustrating a situation where a terahertzelectromagnetic wave generator comprised of Bi and SiO₂ is arranged sothat a left half of the area irradiated with a laser beam will be Bi anda right half of that area will be SiO₂.

FIG. 5B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and SiO₂ was irradiatedwith a femtosecond laser beam and was arranged so that a left half ofthe area irradiated with the laser beam would be Bi and a right half ofthat area would be SiO₂.

FIG. 6 is a graph showing the power spectrum of the terahertzelectromagnetic wave shown in FIG. 3B.

FIG. 7 is a graph showing a variation in the peak intensity of aterahertz electromagnetic wave that a terahertz electromagnetic wavegenerator comprised of Bi and SiO₂ generated when the area irradiatedwith a femtosecond laser beam was scanned one-dimensionally.

FIG. 8A is a perspective view illustrating a situation where a terahertzelectromagnetic wave generator comprised of Bi₂O₃ and SiO₂ is arrangedso that a right half of the area irradiated with a laser beam will beBi₂O₃ and a left half of that area will be SiO₂.

FIG. 8B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂O₃ and SiO₂ wasirradiated with a femtosecond laser beam and was arranged so that aright half of the area irradiated with the laser beam would be Bi₂O₃ anda left half of that area would be SiO₂.

FIG. 9 is a graph showing the power spectrum of the terahertzelectromagnetic wave shown in FIG. 8B.

FIG. 10A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi₂O₃ and SiO₂ isarranged so that a left half of the area irradiated with a laser beamwill be Bi₂O₃ and a right half of that area will be SiO₂.

FIG. 10B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂O₃ and SiO₂ wasirradiated with a femtosecond laser beam and was arranged so that a lefthalf of the area irradiated with the laser beam would be Bi₂O₃ and aright half of that area would be SiO₂.

FIG. 11 is a graph showing the power spectrum of the terahertzelectromagnetic wave shown in FIG. 10B.

FIG. 12 is a graph showing a variation in the peak intensity of aterahertz electromagnetic wave that a terahertz electromagnetic wavegenerator comprised of Bi₂O₃ and SiO₂ generated when the area irradiatedwith a femtosecond laser beam was scanned one-dimensionally.

FIG. 13A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi and Al₂O₃ isarranged so that a right half of the area irradiated with a laser beamwill be Bi and a left half of that area will be Al₂O₃.

FIG. 13B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and Al₂O₃ was irradiatedwith a femtosecond laser beam and was arranged so that a right half ofthe area irradiated with the laser beam would be Bi and a left half ofthat area would be Al₂O₃.

FIG. 14A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi and Al₂O₃ isarranged so that a left half of the area irradiated with a laser beamwill be Bi and a right half of that area will be Al₂O₃.

FIG. 14B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and Al₂O₃ was irradiatedwith a femtosecond laser beam and was arranged so that a left half ofthe area irradiated with the laser beam would be Bi and a right half ofthat area would be Al₂O₃.

FIG. 15A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi and MgO isarranged so that a right half of the area irradiated with a laser beamwill be Bi and a left half of that area will be MgO.

FIG. 15B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and MgO was irradiatedwith a femtosecond laser beam and was arranged so that a right half ofthe area irradiated with the laser beam would be Bi and a left half ofthat area would be MgO.

FIG. 16A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi and MgO isarranged so that a left half of the area irradiated with a laser beamwill be Bi and a right half of that area will be MgO.

FIG. 16B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi and MgO was irradiatedwith a femtosecond laser beam and was arranged so that a left half ofthe area irradiated with the laser beam would be Bi and a right half ofthat area would be MgO.

FIG. 17A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi₂Te₃ and Al₂O₃is arranged so that a right half of the area irradiated with a laserbeam will be Bi₂Te₃ and a left half of that area will be Al₂O₃.

FIG. 17B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂Te₃ and Al₂O₃ wasirradiated with a femtosecond laser beam and was arranged so that aright half of the area irradiated with the laser beam would be Bi₂Te₃and a left half of that area would be Al₂O₃.

FIG. 18A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi₂Te₃ and Al₂O₃is arranged so that a left half of the area irradiated with a laser beamwill be Bi₂Te₃ and a right half of that area will be Al₂O₃.

FIG. 18B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂Te₃ and Al₂O₃ wasirradiated with a femtosecond laser beam and was arranged so that a lefthalf of the area irradiated with the laser beam would be Bi₂Te₃ and aright half of that area would be Al₂O₃.

FIG. 19A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi₂Te₃ and MgO isarranged so that a right half of the area irradiated with a laser beamwill be Bi₂Te₃ and a left half of that area will be MgO.

FIG. 19B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂Te₃ and MgO wasirradiated with a femtosecond laser beam and was arranged so that aright half of the area irradiated with the laser beam would be Bi₂Te₃and a left half of that area would be MgO.

FIG. 20A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Bi₂Te₃ and MgO isarranged so that a left half of the area irradiated with a laser beamwill be Bi₂Te₃ and a right half of that area will be MgO.

FIG. 20B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Bi₂Te₃ and MgO wasirradiated with a femtosecond laser beam and was arranged so that a lefthalf of the area irradiated with the laser beam would be Bi₂Te₃ and aright half of that area would be MgO.

FIG. 21A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Au and SiO₂ isarranged so that a right half of the area irradiated with a laser beamwill be Au and a left half of that area will be SiO₂.

FIG. 21B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Au and SiO₂ was irradiatedwith a femtosecond laser beam and was arranged so that a right half ofthe area irradiated with the laser beam would be Au and a left half ofthat area would be SiO₂.

FIG. 22A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Au and SiO₂ isarranged so that a left half of the area irradiated with a laser beamwill be Au and a right half of that area will be SiO₂.

FIG. 22B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Au and SiO₂ was irradiatedwith a femtosecond laser beam and was arranged so that a left half ofthe area irradiated with the laser beam would be Au and a right half ofthat area would be SiO₂.

FIG. 23A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Fe₂O₃ and Al₂O₃ isarranged so that a right half of the area irradiated with a laser beamwill be Fe₂O₃ and a left half of that area will be Al₂O₃.

FIG. 23B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Fe₂O₃ and Al₂O₃ wasirradiated with a femtosecond laser beam and was arranged so that aright half of the area irradiated with the laser beam would be Fe₂O₃ anda left half of that area would be Al₂O₃.

FIG. 24A is a perspective view illustrating a situation where aterahertz electromagnetic wave generator comprised of Fe₂O₃ and Al₂O₃ isarranged so that a left half of the area irradiated with a laser beamwill be Fe₂O₃ and a right half of that area will be Al₂O₃.

FIG. 24B is a graph showing the time domain waveform of a terahertzelectromagnetic wave generated in such a situation where the terahertzelectromagnetic wave generator comprised of Fe₂O₃ and Al₂O₃ wasirradiated with a femtosecond laser beam and was arranged so that a lefthalf of the area irradiated with the laser beam would be Fe₂O₃ and aright half of that area would be Al₂O₃.

DETAILED DESCRIPTION

A terahertz electromagnetic wave generator according to the presentdisclosure uses the Seebeck effect to be expressed by a thermoelectricmaterial. The Seebeck effect is a phenomenon that a difference intemperature in an object is directly converted to a voltage and is akind of a thermoelectric effect. FIG. 1A schematically illustrates howthe Seebeck effect is produced. In FIG. 1A, illustrated are an n-typethermoelectric material and a p-type thermoelectric material. In each ofthese materials, the temperature is higher at its left end than at itsright end. In this case, in the n-type thermoelectric material,electrons that are majority carriers move (i.e., diffuse thermally) fromthe left at a relatively high temperature to the right at a relativelylow temperature, thus generating a voltage. On the other hand, in thep-type thermoelectric material, holes that are majority carriers move(i.e., diffuse thermally) from the left at a relatively high temperatureto the right at a relatively low temperature, thus generating a voltage.In both of these two materials, the majority carriers move in the samedirection from a portion at the higher temperature toward a portion atthe lower temperature. However, the polarity of the majority carriers ofthe n-type thermoelectric material is opposite from that of the majoritycarriers of the p-type thermoelectric material, and therefore, currentsflow through the materials in mutually opposite directions.

In general, a thermoelectric material is a material which generates avoltage and current by producing a temperature gradient within thesubstance. According to the present disclosure, a temperature gradientis introduced into a thermoelectric material with a femtosecond laserbeam, thereby producing current through the thermal diffusion. And byusing that current, a terahertz electromagnetic wave is generated bydipole radiation. Nevertheless, if current has been inducedsymmetrically in a space, it can be said that no current has beenproduced macroscopically, and therefore, no terahertz electromagneticwave is generated, either. On the other hand, if asymmetric current canbe induced, then the current will flow in one direction macroscopically,and a terahertz electromagnetic wave can be generated by dipoleradiation.

A terahertz electromagnetic wave generator according to the presentdisclosure includes: a substrate; a thermoelectric material layer whichis supported by the substrate and which has a surface; and a pulsedlaser light source system which locally heats the thermoelectricmaterial layer with an edge of the surface of the thermoelectricmaterial layer irradiated with pulsed light, thereby generating aterahertz electromagnetic wave from the thermoelectric material layer.

Embodiments of the present disclosure will now be described.

EMBODIMENTS

FIG. 1B is a perspective view illustrating a terahertz electromagneticwave generator 4 as an embodiment of the present disclosure. As shown inFIG. 1B, the terahertz electromagnetic wave generator 4 for use in thisembodiment includes a substrate 1 and a thermoelectric material layer 2which is supported by the substrate 1. For your reference, XYZcoordinates represented by X, Y and Z axes that intersect with eachother at right angles are shown in FIG. 1B. The substrate 1 illustratedin FIG. 1B has a flat plate shape. The principal surface of thesubstrate 1 is parallel to an XY plane and intersects with the Z axis atright angles.

FIG. 1C is a perspective view schematically illustrating an exemplaryconfiguration for a terahertz electromagnetic wave generator accordingto an embodiment of the present disclosure. As shown in FIG. 1C, theterahertz electromagnetic wave generator of this embodiment includes afemtosecond laser light source 3 and a terahertz electromagnetic wavegenerator 4, which includes the thermoelectric material layer 2supported by the substrate 1 as described above. The femtosecond laserlight source 3 of this embodiment irradiates an edge of thethermoelectric material layer 2 of the terahertz electromagnetic wavegenerator 4 with a pulsed femtosecond laser beam 5. The femtosecondlaser beam may have a pulse width of 1 femtosecond to 1 nanosecond, andtypically has a pulse width falling within the range of 10 femtosecondsto 100 femtoseconds. Such a pulsed femtosecond laser beam can beradiated 1 to 10⁸ times per second. At the edge of the thermoelectricmaterial layer 2 irradiated with such a pulsed femtosecond laser beam,the temperature rises in a short time (which is approximately as long asa laser beam radiation time) and current is thermally diffused due tothe Seebeck effect and flows inward from the edge of the thermoelectricmaterial layer 2. And based on this current, a terahertz electromagneticwave 6 is radiated from around the edge of the thermoelectric materiallayer 2 into the surrounding space.

FIG. 2A is a cross-sectional view schematically illustrating how theterahertz electromagnetic wave generator 4 operates when a femtosecondlaser beam 5 is incident on the generator 4, and FIG. 2B is a schematictop view of the generator 4. In the terahertz electromagnetic wavegenerator 4 shown in FIGS. 2A and 2B, the temperature rises steeply in ashort time in its portion irradiated with the femtosecond laser beam 5,and a temperature gradient is produced generally in the directionindicated by the open arrow 30 in FIG. 2A. The temperature gradient hasnot only a component which is parallel to the X-axis direction shown inFIG. 2B but also a component in the Y-axis direction as well. However,in the temperature gradient, components in the +Y-axis and −Y-axisdirections may be symmetric to each other, but components in the +X-axisand −X-axis directions are not symmetric to each other. Such atemperature gradient is produced by selectively raising the temperatureof the thermoelectric material layer 2 only at its edge.

FIG. 2C is a top view schematically illustrating how the terahertzelectromagnetic wave generator 4 operates when a center portion of thesurface 20 of the thermoelectric material layer 2 is irradiated with thefemtosecond laser beam 5. In this case, the spot of the femtosecondlaser beam 5 does not cross the edge 20 e of the surface 20 of thethermoelectric material layer 2, and the thermoelectric material layer 2is irradiated with the femtosecond laser beam 5 fully. As schematicallyindicated by the arrows 30 in FIG. 2C, the temperature gradient producedin this case is isotropic within the XY plane.

The present inventors discovered via experiments that when thetemperature gradient was isotropic within the XY plane, no terahertzelectromagnetic wave was generated (as will be described later). In anembodiment of the present disclosure, on the other hand, a terahertzelectromagnetic wave is generated. This is probably because thermallydiffused current is produced asymmetrically within the XY plane as shownin FIG. 2B, for example.

According to this embodiment, if the spot radius of the femtosecondlaser beam 5 is r, the radiation position of the femtosecond laser beamis adjusted so that the edge 20 e of the surface 20 of thethermoelectric material layer 2 is located within the distance r fromthe center of the laser beam, and the surface edge of the thermoelectricmaterial layer 2 is selectively heated by being irradiated with a pulsedlaser beam. The spot radius r of the femtosecond laser beam 5 is definedto be the radius of an area where the beam intensity becomes equal to orgreater than 1/e of the beam center intensity, where e is the base ofnatural logarithms and is represented by an approximate value of 2.7.

The femtosecond laser beam for use in this embodiment raises thetemperature of the thermoelectric material layer 2 with pulses, andtherefore, the wavelength of the laser beam is set to be a value fallingwithin the range in which the laser beam is absorbed into thethermoelectric material layer 2. The wavelength range in which the laserbeam is absorbed into the thermoelectric material layer 2 may varyaccording to the type of the thermoelectric material that forms thethermoelectric material layer 2.

In the terahertz electromagnetic wave generator of the presentdisclosure, the thermally diffused current produced by the Seebeckeffect becomes the source of a terahertz electromagnetic wave, andtherefore, it does not matter whether the material of the thermoelectricmaterial layer 2 is an n-type material or a p-type one. Thethermoelectric material layer 2 may be made of a material with a largeSeebeck coefficient and a high degree of electrical conductivity.Examples of thermoelectric materials which may be used to make thethermoelectric material layer 2 include single-element thermoelectricmaterials such as Bi and Sb, alloy-based thermoelectric materials suchas BiTe, PbTe and SiGe based materials, and oxide-based thermoelectricmaterials such as Ca_(x)CoO₂, Na_(x)CoO₂, and SrTiO₃. In thisdescription, the “thermoelectric material” refers herein to a materialwith a Seebeck coefficient, of which the absolute value is equal to orgreater than 30 μV/K, and an electrical resistivity of 10 mΩcm or less.Such a thermoelectric material may be either crystalline or amorphous.

The substrate 1 of this embodiment is made of a material which cantransmit the terahertz electromagnetic wave generated and may be made ofa dielectric material, for example. Examples of dielectric materialswhich may be used to make the substrate 1 include SiO₂, Al₂O₃, MgO, Siand LSAT. Not the entire substrate 1 has to be made of the samematerial, and the substrate 1 does not have to have a uniform thickness,either. The principal surface of the substrate 1 is typically flat butmay have some unevenness, too. The function to be performed by thesubstrate 1 is to support the thermoelectric material layer 2. As longas the substrate 1 can perform this function, the substrate 1 may haveany of various forms.

In the terahertz electromagnetic wave generator 4 of the presentdisclosure, the thermoelectric material layer 2 may have a thickness atwhich 50% or more of the terahertz electromagnetic wave generated can betransmitted. In the area where the terahertz electromagnetic wave isgenerated, the thickness of the thermoelectric material layer 2 may beset to fall within the range of 10 nm to 1000 nm (=1 μm), for example.The thermoelectric material layer 2 may be formed by any method, whichmay be a sputtering process, an evaporation process, a laser ablationprocess, a vapor deposition process such as chemical vapor deposition, aliquid phase deposition process, or any of various other methods. Thethermoelectric material layer 2 does not have to be deposited directlyon the principal surface of the substrate 1. Alternatively, thethermoelectric material layer 2 may be deposited on another substrateand then transferred onto the principal surface of the substrate 1.

FIG. 2D illustrates an exemplary configuration for a terahertzspectrometer according to an embodiment of the present disclosure. Thisspectrometer includes a light source system 100 which emits thefemtosecond laser beam 5, the terahertz electromagnetic wave generator 4of the present disclosure, an optical system (including mirrors 200 aand 200 b) which irradiates an object (sample 300) with the terahertzelectromagnetic wave 6 generated by the terahertz electromagnetic wavegenerator 4, and a detector 400 which detects the terahertzelectromagnetic wave 16 that is transmitted through the sample 300.Optionally, this terahertz spectrometer may further include a processingapparatus 500 which generates an image representing a terahertzelectromagnetic wave with a particular wavelength based on the output ofthe detector 400.

As shown in FIG. 2E, a method of generating a terahertz electromagneticwave according to the present disclosure includes the step S100 ofproviding a thermoelectric material body and the step S200 of locallyheating the thermoelectric material body by irradiating an edge of thethermoelectric material body with pulsed light (e.g., the femtosecondlaser beam). This step S200 includes the step S210 of locally heatingthe thermoelectric material body so that an asymmetric heat distributionis formed in the thermoelectric material body, and the step S220 ofproducing thermal diffusion current in the portion of the thermoelectricmaterial body that is heated locally, thereby generating a terahertzelectromagnetic wave.

The thermoelectric material layer 2 for use in this embodiment does nothave to have the shape shown in FIGS. 2A and 2B. Alternatively, thethermoelectric material layer 2 may also be patterned by a known methodto have any other shape. For example, the thermoelectric material layer2 may have a linear pattern, a bent curved pattern or a curvilinearpattern or may also have a single or a plurality of holes. Thethermoelectric material layer 2 may be divided into a plurality ofportions on the single substrate 1 or may cover the principal surface ofthe substrate 1 entirely. Optionally, the thermoelectric material layer2 may be partially extended out of the principal surface of thesubstrate 1. The thermoelectric material layer 2 may also be a nanowirelayer. The surface of the thermoelectric material layer 2 does not haveto be flat and its thickness does not have to be uniform within theplane, either.

FIG. 2F is a cross-sectional view schematically illustrating a terahertzelectromagnetic wave generator 4 with a thermoelectric material layer 2which has been patterned into a linear shape, and FIG. 2G is a schematictop view thereof. In the terahertz electromagnetic wave generator 4illustrated in FIGS. 2F and 2G, an edge 20 e of the surface 20 of thethermoelectric material layer 2 that is supported on the substrate 1 isalso irradiated with a femtosecond laser beam 5. In the exampleillustrated in FIGS. 2F and 2G, one of the three linear portions intowhich the thermoelectric material layer 2 has been patterned has one ofits edges irradiated with the femtosecond laser beam 5.

FIG. 2H is a cross-sectional view schematically illustrating a terahertzelectromagnetic wave generator 4 including a thermoelectric materiallayer 2 with a hole, and FIG. 2I is a schematic top view thereof. In theterahertz electromagnetic wave generator 4 illustrated in FIGS. 2H and2I, an edge 20 e of the surface 20 of the thermoelectric material layer2 is also irradiated with a femtosecond laser beam 5. In the exampleillustrated in FIGS. 2H and 2I, the edge 20 e of the surface 20 is aportion of the outer periphery of the hole.

The following are some specific examples of the present disclosure.

EXAMPLE 1

An element which used Bi as a thermoelectric material and SiO₂ as asubstrate material, respectively, was fabricated by the followingmethod. Bi is an n-type thermoelectric material and had a Seebeckcoefficient of −75 μV/K and an electrical resistivity of 0.1 mΩcm. Onthe other hand, SiO₂ is an insulator, not a thermoelectric material.

A Bi layer was deposited by evaporation process to a thickness of 50 nmon an SiO₂ substrate (10 mm×10 mm×0.5 mm). The evaporation process wascarried out without heating the SiO₂ substrate after the film depositionchamber had been evacuated to a pressure of 1.0×10⁻³ Pa or less. Whenthe Bi layer was deposited, the principal surface of the SiO₂ substratewas covered with a metallic mask so that only a portion of the principalsurface of the SiO₂ substrate would be coated with the Bi layer. In thismanner, a terahertz electromagnetic wave generator with the structureshown in FIG. 1B (i.e., comprised of a thermoelectric material (Bi) andan insulator (SiO₂)) was fabricated. While the SiO₂ substrate had anarea of 10 mm×10 mm, the Bi layer deposited on the substrate had an areaof 1 mm×1 mm.

As the femtosecond laser light source to heat the terahertzelectromagnetic wave generator, a Ti: Sapphire laser diode with awavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHzwas used.

With respect to the terahertz electromagnetic wave generator thusfabricated, one edge of the Bi layer was irradiated with a femtosecondlaser beam that had been condensed to 100 μm (=2r), thereby measuringthe time domain waveform of the terahertz electromagnetic wave. In thiscase, adjustment was made so that the center of the laser beam would belocated on one edge of the Bi layer. First of all, the terahertzelectromagnetic wave generator was irradiated with the femtosecond laserbeam so that a right half of the area irradiated with the laser beamwould be Bi and a left half of that area would be SiO₂ as shown in FIG.3A. The time domain waveform of the terahertz electromagnetic wavegenerated in such a situation is shown in FIG. 3B. As can be seen fromthe time domain waveform of the terahertz electromagnetic wave shown inFIG. 3B, a pulse wave with a negative peak intensity was generated ataround 20 ps (picoseconds). FIG. 4 shows a power spectrum obtained bysubjecting this time domain waveform to a Fourier transform. Theelectromagnetic wave generated had a frequency range of about 0.1 THz toabout 1 THz. Thus, it was confirmed that a terahertz electromagneticwave had been generated.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be SiO₂ and a left half of that area would beBi as shown in FIG. 5A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 5B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 5B, a pulse wave with a positive peakintensity was generated at around 20 ps (picoseconds). FIG. 6 shows apower spectrum obtained by subjecting this time domain waveform to aFourier transform. The electromagnetic wave generated had a frequencyrange of about 0.1 THz to about 1 THz. Thus, it was confirmed that aterahertz electromagnetic wave had been generated.

As can be seen from these results, the electric field polarity of theterahertz electromagnetic wave (i.e., whether the maximum peak of thetime waveform is positive or negative) inverted depending on whether theterahertz electromagnetic wave had been generated so that a right halfof the area irradiated with the laser beam would be Bi and a left halfof that area would be SiO₂ as shown in FIG. 3A or that a right half ofthe area irradiated with the laser beam would be SiO₂ and a left half ofthat area would be Bi as shown in FIG. 5A. The electric field polarityof the terahertz electromagnetic wave is determined by the direction ofcurrent to be its source. That is why if the electric field polarity ofthe terahertz electromagnetic wave observed in FIG. 3B was inverse ofthat of the terahertz electromagnetic wave observed in FIG. 5B, then itmeans that the direction of current to be induced changed depending onwhether Bi was located on the left half of the area irradiated with thelaser beam or on the right half thereof. In fact, if the terahertzelectromagnetic wave generator is irradiated with the femtosecond laserbeam so that a right half of the area irradiated with the laser beamwill be Bi and a left half of that area will be SiO₂ as shown in FIG.3A, electrons heated within Bi will diffuse to the left, and therefore,current will flow to the right. On the other hand, if the terahertzelectromagnetic wave generator is irradiated with the femtosecond laserbeam so that a right half of the area irradiated with the laser beamwill be SiO₂ and a left half of that area will be Bi as shown in FIG.5A, electrons heated within Bi will diffuse to the right, and therefore,current will flow to the left. Since SiO₂ is an insulator, no currentwill flow through SiO₂ due to thermal diffusion.

Thus it can be understood why the polarity of the terahertzelectromagnetic wave observed in FIG. 3B was inverse of that of theterahertz electromagnetic wave observed in FIG. 5B. And it can also beseen that the current generated in Bi that is a thermoelectric materialbecomes the source of a terahertz electromagnetic wave.

Next, a variation in the peak intensity of the terahertz electromagneticwave generated was traced by scanning the area irradiated with thefemtosecond laser beam from the SiO₂ side toward the Bi side. In thiscase, the femtosecond laser beam had a beam diameter (=2r) of 100 μm.The results are shown in FIG. 7. When the laser beam spot was locatedcompletely within the SiO₂ area, no terahertz electromagnetic wave wasgenerated. However, when the laser beam was incident on an edge of theBi layer, the peak intensity of the terahertz electromagnetic wavestarted to rise. And when the center of the laser beam spot was locatedin the vicinity of an edge of the Bi layer, the peak intensity of theterahertz electromagnetic wave reached its maximum value. As the laserbeam spot was further shifted toward the Bi side, the peak intensity ofthe terahertz electromagnetic wave started to decrease. And when thelaser beam spot entered the Bi area completely, no terahertzelectromagnetic wave was observed anymore. However, when the laser beamspot was shifted to reach the opposite edge of the Bi layer, a terahertzelectromagnetic wave was observed again. These results reveal that togenerate a terahertz electromagnetic wave, it is important that an edgeof the thermoelectric material layer falls within the area irradiatedwith the laser beam.

EXAMPLE 2

An element which used p-type Bi₂Te₃ as a thermoelectric material andSiO₂ as a substrate material, respectively, was fabricated by thefollowing method. Bi₂Te₃ had a Seebeck coefficient of +210 μV/K and anelectrical resistivity of 1 mΩcm. On the other hand, SiO₂ is aninsulator, not a thermoelectric material.

A Bi₂Te₃ layer was deposited by evaporation process to a thickness of 50nm on an SiO₂ substrate (10 mm×10 mm×0.5 mm). The evaporation processwas carried out without heating the SiO₂ substrate after the filmdeposition chamber had been evacuated to a pressure of 1.0×10⁻³ Pa orless. When the Bi₂Te₃ layer was deposited, the principal surface of theSiO₂ substrate was covered with a metallic mask so that only a portionof the principal surface of the SiO₂ substrate would be coated with theBi₂Te₃ layer. In this manner, a terahertz electromagnetic wave generatorwith the structure shown in FIG. 1B (i.e., comprised of a thermoelectricmaterial (Bi₂Te₃) and an insulator (SiO₂)) was fabricated. While theSiO₂ substrate had an area of 10 mm×10 mm, the Bi₂Ti₃ layer deposited onthe substrate had an area of 1 mm×1 mm.

As a femtosecond laser diode to heat the terahertz electromagnetic wavegenerator, a Ti: Sapphire laser diode with a wavelength of 800 nm, apulse width of 100 fs and a pulse rate of 80 MHz was used.

With respect to the terahertz electromagnetic wave generator thusfabricated, one edge of the Bi₂Te₃ layer was irradiated with afemtosecond laser beam that had been condensed to 100 μm (=2r), therebymeasuring the time domain waveform of the terahertz electromagneticwave. In this case, adjustment was made so that the center of the laserbeam would be located on one edge of the Bi₂Ti₃ layer.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Bi₂Te₃ and a left half ofthat area would be SiO₂ as shown in FIG. 8A. The time domain waveform ofthe terahertz electromagnetic wave generated in such a situation isshown in FIG. 8B. As can be seen from the time domain waveform of theterahertz electromagnetic wave shown in FIG. 8B, a pulse wave with apositive peak intensity was generated at around 20 ps. FIG. 9 shows apower spectrum obtained by subjecting this time domain waveform to aFourier transform. The electromagnetic wave generated had a frequencyrange of about 0.1 THz to about 1 THz. Thus, it was confirmed that aterahertz electromagnetic wave had been generated.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be SiO₂ and a left half of that area would beBi₂Te₃ as shown in FIG. 10A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 10B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 10B, a pulse wave with a negativepeak intensity was generated at around 20 ps. FIG. 11 shows a powerspectrum obtained by subjecting this time domain waveform to a Fouriertransform. The electromagnetic wave generated had a frequency range ofabout 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertzelectromagnetic wave had been generated.

As can be seen from these results, the electric field polarity of theterahertz electromagnetic wave (i.e., whether the maximum peak of thetime waveform is positive or negative) inverted depending on whether theterahertz electromagnetic wave had been generated so that a right halfof the area irradiated with the laser beam would be Bi₂Te₃ and a lefthalf of that area would be SiO₂ as shown in FIG. 8A or that a right halfof the area irradiated with the laser beam would be SiO₂ and a left halfof that area would be Bi₂Te₃ as shown in FIG. 10A. The electric fieldpolarity of the terahertz electromagnetic wave is determined by thedirection of current to be its source. That is why if the electric fieldpolarity of the terahertz electromagnetic wave observed in FIG. 8B wasinverse of that of the terahertz electromagnetic wave observed in FIG.10B, then it means that the direction of current to be induced changeddepending on whether Bi₂Te₃ was located on the left half of the areairradiated with the laser beam or on the right half thereof. In fact, ifthe terahertz electromagnetic wave generator is irradiated with thefemtosecond laser beam so that a left half of the area irradiated withthe laser beam will be Bi₂Te₃ and a right half of that area will be SiO₂as shown in FIG. 10A, holes heated within Bi₂Te₃ will diffuse to theleft, and therefore, current will flow to the left. On the other hand,if the terahertz electromagnetic wave generator is irradiated with thefemtosecond laser beam so that a left half of the area irradiated withthe laser beam will be SiO₂ and a right half of that area will be Bi₂Te₃as shown in FIG. 8A, holes heated within Bi₂Te₃ will diffuse to theright, and therefore, current will flow to the right. This behavior isopposite from that of Example 1 in which Bi was used as an n-typethermoelectric material, and indicates the difference in the type ofcarriers (i.e., whether the carriers are electrons or holes) betweenthese two examples. Since SiO₂ is an insulator, no current will flowthrough SiO₂ due to thermal diffusion.

Thus it can be understood why the polarity of the terahertzelectromagnetic wave observed in FIG. 8B was inverse of that of theterahertz electromagnetic wave observed in FIG. 10B. And it can also beseen that the current generated in Bi₂Te₃ that is a thermoelectricmaterial becomes the source of a terahertz electromagnetic wave.

Next, a variation in the peak intensity of the terahertz electromagneticwave generated was traced by scanning the area irradiated with thefemtosecond laser beam from the SiO₂ side toward the Bi₂Te₃ side. Inthis case, the femtosecond laser beam had a beam diameter of 100 μm. Theresults are shown in FIG. 12. When the laser beam spot was locatedcompletely within the SiO₂ area, no terahertz electromagnetic wave wasgenerated. However, when the laser beam was incident on an edge of theBi₂Te₃ layer, the peak intensity of the terahertz electromagnetic wavestarted to rise. And when the center of the laser beam spot was locatedin the vicinity of an edge of the Bi₂Te₃ layer, the peak intensity ofthe terahertz electromagnetic wave reached its maximum value. As thelaser beam spot was further shifted toward the Bi₂Te₃ side, the peakintensity of the terahertz electromagnetic wave started to decrease. Andwhen the laser beam spot entered the Bi₂Te₃ area completely, noterahertz electromagnetic wave was observed anymore. However, when thelaser beam spot was shifted to reach the opposite edge of the Bi₂Te₃layer, a terahertz electromagnetic wave was observed again. Theseresults reveal that to generate a terahertz electromagnetic wave, it isimportant that an edge of the thermoelectric material layer falls withinthe area irradiated with the laser beam.

The terahertz electromagnetic wave radiation properties of Bi and Bi₂Te₃that have been described for Examples 1 and 2 exhibited no dependence onthe polarization of the femtosecond laser beam. This indicates that theterahertz electromagnetic wave generating mechanism is not a secondarynonlinear effect. As for semiconductors and insulators, on the otherhand, a so-called “photo-Dember effect” that is a terahertzelectromagnetic wave generating mechanism which has something to do withdiffusion of photo-excited carriers has also been reported. However, thepolarity of a terahertz electromagnetic wave generated under thephoto-Dember effect does not depend on the type of the majority carriers(i.e., whether the majority carriers are electrons or holes) (seePhysical Review B73, 155330, (2006)), which is different from theterahertz electromagnetic wave radiation properties of Examples 1 and 2.Consequently, the terahertz electromagnetic wave generating mechanism ofthe present disclosure does not result from the photo-Dember effect.

As can be seen from the foregoing description, a method of generating aterahertz electromagnetic wave according to the present disclosure isbased on a novel mechanism, and the present disclosure provides a simpleterahertz electromagnetic wave source which needs no external voltagesupply.

EXAMPLE 3

An element which used Bi as a thermoelectric material and Al₂O₃ as asubstrate material, respectively, was fabricated by the followingmethod. Bi is an n-type thermoelectric material and had a Seebeckcoefficient of −75 μV/K and an electrical resistivity of 0.1 mΩcm. Onthe other hand, Al₂O₃ is an insulator, not a thermoelectric material.

The same measurement was made under the same condition as in Example 1.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Bi and a left half of thatarea would be Al₂O₃ as shown in FIG. 13A. The time domain waveform ofthe terahertz electromagnetic wave generated in such a situation isshown in FIG. 13B. As can be seen from the time domain waveform of theterahertz electromagnetic wave shown in FIG. 13B, a pulse wave with anegative peak intensity was generated at around 20 ps.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be Al₂O₃ and a left half of that area would beBi as shown in FIG. 14A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 14B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 14B, a pulse wave with a positivepeak intensity was generated at around 20 ps.

As in Example 1 described above, it can be understood why the polarityof the terahertz electromagnetic wave observed in FIG. 13B was inverseof that of the terahertz electromagnetic wave observed in FIG. 14B. Andit can also be seen that even when Al₂O₃ was used as a substratematerial, a terahertz electromagnetic wave was also generated on thesame principle.

EXAMPLE 4

An element which used Bi as a thermoelectric material and MgO as asubstrate material, respectively, was fabricated by the followingmethod. Bi is an n-type thermoelectric material and had a Seebeckcoefficient of −75 μV/K and an electrical resistivity of 0.1 mΩcm. Onthe other hand, MgO is an insulator, not a thermoelectric material.

The same measurement was made under the same condition as in Example 1.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Bi and a left half of thatarea would be MgO as shown in FIG. 15A. The time domain waveform of theterahertz electromagnetic wave generated in such a situation is shown inFIG. 15B. As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 15B, a pulse wave with a negativepeak intensity was generated at around 20 ps.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be MgO and a left half of that area would beBi as shown in FIG. 16A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 16B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 16B, a pulse wave with a positivepeak intensity was generated at around 20 ps.

As in Example 1 described above, it can be understood why the polarityof the terahertz electromagnetic wave observed in FIG. 15B was inverseof that of the terahertz electromagnetic wave observed in FIG. 16B. Andit can also be seen that even when MgO was used as a substrate material,a terahertz electromagnetic wave was also generated on the sameprinciple.

EXAMPLE 5

An element which used p-type Bi₂Te₃ as a thermoelectric material andAl₂O₃ as a substrate material, respectively, was fabricated by thefollowing method. Bi₂Te₃ had a Seebeck coefficient of +210 μV/K and anelectrical resistivity of 1 mΩcm. On the other hand, Al₂O₃ is aninsulator, not a thermoelectric material. The same measurement was madeunder the same condition as in Example 1.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Bi₂Te₃ and a left half ofthat area would be Al₂O₃ as shown in FIG. 17A. The time domain waveformof the terahertz electromagnetic wave generated in such a situation isshown in FIG. 17B. As can be seen from the time domain waveform of theterahertz electromagnetic wave shown in FIG. 17B, a pulse wave with apositive peak intensity was generated at around 20 ps.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be Al₂O₃ and a left half of that area would beBi₂Te₃ as shown in FIG. 18A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 18B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 18B, a pulse wave with a negativepeak intensity was generated at around 20 ps.

As in Example 4 described above, it can be understood why the polarityof the terahertz electromagnetic wave observed in FIG. 17B was inverseof that of the terahertz electromagnetic wave observed in FIG. 18B. Andit can also be seen that even when Al₂O₃ was used as a substratematerial, a terahertz electromagnetic wave was also generated on thesame principle.

EXAMPLE 6

An element which used p-type Bi₂Te₃ as a thermoelectric material and MgOas a substrate material, respectively, was fabricated by the followingmethod. Bi₂Te₃ had a Seebeck coefficient of +210 μV/K and an electricalresistivity of 1 mΩcm. On the other hand, MgO is an insulator, not athermoelectric material. The same measurement was made under the samecondition as in Example 1.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Bi₂Te₃ and a left half ofthat area would be MgO as shown in FIG. 19A. The time domain waveform ofthe terahertz electromagnetic wave generated in such a situation isshown in FIG. 19B. As can be seen from the time domain waveform of theterahertz electromagnetic wave shown in FIG. 19B, a pulse wave with apositive peak intensity was generated at around 20 ps.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be MgO and a left half of that area would beBi₂Te₃ as shown in FIG. 20A. The time domain waveform of the terahertzelectromagnetic wave generated in such a situation is shown in FIG. 20B.As can be seen from the time domain waveform of the terahertzelectromagnetic wave shown in FIG. 20B, a pulse wave with a negativepeak intensity was generated at around 20 ps.

As in Example 4 described above, it can be understood why the polarityof the terahertz electromagnetic wave observed in FIG. 19B was inverseof that of the terahertz electromagnetic wave observed in FIG. 20B. Andit can also be seen that even when MgO was used as a substrate material,a terahertz electromagnetic wave was also generated on the sameprinciple.

COMPARATIVE EXAMPLE 1

As a comparative example, the same experiment was carried out on Auwhich is not a thermoelectric material (and which had a Seebeckcoefficient of +2 μV/K and an electrical resistivity of 0.002 mΩcm).

An Au layer was deposited by evaporation process to a thickness of 50 nmon an SiO₂ substrate (10 mm×10 mm×0.5 mm). The evaporation process wascarried out without heating the SiO₂ substrate after the film depositionchamber had been evacuated to a pressure of 1.0×10⁻³ Pa or less. Whenthe Au layer was deposited, the principal surface of the SiO₂ substratewas covered with a metallic mask so that only a portion of the principalsurface of the SiO₂ substrate would be coated with the Au layer. Whilethe SiO₂ substrate had an area of 10 mm×10 mm, the Au layer deposited onthe substrate had an area of 1 mm×1 mm.

As a femtosecond laser diode to heat the terahertz electromagnetic wavegenerator, a Ti: Sapphire laser diode with a wavelength of 800 nm, apulse width of 100 fs and a pulse rate of 80 MHz was used.

With respect to the terahertz electromagnetic wave generator thusfabricated, one edge of the Au layer was irradiated with a femtosecondlaser beam that had been condensed to 100 μm (=2r), thereby measuringthe time domain waveform of the terahertz electromagnetic wave. In thiscase, adjustment was made so that the center of the laser beam would belocated on one edge of the Au layer.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Au and a left half of thatarea would be SiO₂ as shown in FIG. 21A. The time domain waveform of theelectromagnetic wave generated in such a situation is shown in FIG. 21B.As can be seen from the time domain waveform of the electromagnetic waveshown in FIG. 21B, there was nothing but noise with no definite peaksobserved, and no terahertz electromagnetic wave had been generated.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be SiO₂ and a left half of that area would beAu as shown in FIG. 22A. The time domain waveform of the electromagneticwave generated in such a situation is shown in FIG. 22B. As can be seenfrom the time domain waveform of the electromagnetic wave shown in FIG.22B, there was nothing but noise with no definite peaks observed, and noterahertz electromagnetic wave had been generated.

Thus, the present inventors discovered that no terahertz electromagneticwave could be generated even if the element of the present disclosurewas made of Au that is not a thermoelectric material defined herein,because Au certainly has low electrical resistivity but its Seebeckcoefficient is also low.

COMPARATIVE EXAMPLE 2

As a comparative example, the same experiment was carried out on Fe₂O₃which is not a thermoelectric material (and which had a Seebeckcoefficient of −300 μV/K and an electrical resistivity of 10⁵ mΩcm).

An Fe₂O₃ layer was deposited by evaporation process to a thickness of 50nm on an Al₂O₃ substrate (10 mm×10 mm×0.5 mm). The evaporation processwas carried out without heating the Al₂O₃ substrate after the filmdeposition chamber had been evacuated to a pressure of 1.0×10⁻³ Pa orless. When the Fe₂O₃ layer was deposited, the principal surface of theAl₂O₃ substrate was covered with a metallic mask so that only a portionof the principal surface of the Al₂O₃ substrate would be coated with theFe₂O₃ layer. While the Al₂O₃ substrate had an area of 10 mm×10 mm, theFe₂O₃ layer deposited on the substrate had an area of 1 mm×1 mm.

As a femtosecond laser diode to heat the terahertz electromagnetic wavegenerator, a Ti: Sapphire laser diode with a wavelength of 800 nm, apulse width of 100 fs and a pulse rate of 80 MHz was used.

With respect to the terahertz electromagnetic wave generator thusfabricated, one edge of the Fe₂O₃ layer was irradiated with afemtosecond laser beam that had been condensed to 100 μm (=2r), therebymeasuring the time domain waveform of the terahertz electromagneticwave. In this case, adjustment was made so that the center of the laserbeam would be located on one edge of the Fe₂O₃ layer and Al₂O₃.

First of all, the terahertz electromagnetic wave generator wasirradiated with the femtosecond laser beam so that a right half of thearea irradiated with the laser beam would be Fe₂O₃ and a left half ofthat area would be Al₂O₃ as shown in FIG. 23A. The time domain waveformof the electromagnetic wave generated in such a situation is shown inFIG. 23B. As can be seen from the time domain waveform of theelectromagnetic wave shown in FIG. 23B, there was nothing but noise withno definite peaks observed, and no terahertz electromagnetic wave hadbeen generated.

Next, the terahertz electromagnetic wave generator was irradiated withthe femtosecond laser beam so that a right half of the area irradiatedwith the laser beam would be Al₂O₃ and a left half of that area would beFe₂O₃ as shown in FIG. 24A. The time domain waveform of theelectromagnetic wave generated in such a situation is shown in FIG. 24B.As can be seen from the time domain waveform of the electromagnetic waveshown in FIG. 24B, there was nothing but noise with no definite peaksobserved, and no terahertz electromagnetic wave had been generated.

Thus, the present inventors discovered that no terahertz electromagneticwave could be generated even if the element of the present disclosurewas made of Fe₂O₃ that is not a thermoelectric material defined herein,because Fe₂O₃ certainly has a high Seebeck coefficient but itselectrical resistivity is also high.

In the examples described above, the thermoelectric material layer issupposed to be irradiated with the femtosecond laser beam so that itsbeam spot crosses an edge of the surface of the thermoelectric materiallayer on the right- or left-hand side (i.e., an edge X which runsparallel to the Y axis of an XY plane). However, this is just an exampleand the laser beam spot does not always have to be formed there.Alternatively, the laser beam spot may also be formed across an upper orlower edge of the surface of the thermoelectric material layer (i.e., anedge which runs parallel to the X axis of the XY plane), for example.Also, since the thermoelectric material layer may be patterned into anyarbitrary shape, the location and direction of the edge do not have tobe the exemplary ones of the examples described above, either.

Also, in the examples described above, the thickness of thethermoelectric material layer is set to be 50 nm to make the comparisoneasily. However, it would be obvious to those skilled in the art fromthe entire disclosure of the present application that even if thethickness of the thermoelectric material layer is not 50 nm but fallswithin the range of 10 nm to 1000 nm (=1 μm), for example, similareffects will also be achieved.

Furthermore, as can be seen easily from the principle of generating aterahertz electromagnetic wave that has already been described in theforegoing description of the present disclosure, respective materialsfor the thermoelectric material layer and substrate according to thepresent disclosure do not have to be the ones used in the examplesdescribed above, but any of a wide variety of thermoelectric materialsand substrate materials may be used in an arbitrary combination.

With the technique of generating a terahertz electromagnetic waveaccording to the present disclosure, a terahertz electromagnetic wavecan be generated by using a simpler configuration than conventional oneswithout providing any external voltage supply. Thus, the presentdisclosure is applicable to not only evaluation of the properties ofvarious kinds of materials, but also security, healthcare and numerousother fields, using a terahertz electromagnetic wave.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A terahertz electromagnetic wave generatorcomprising: a substrate; a thermoelectric material layer supported bythe substrate, the thermoelectric material layer having a surface; and apulsed laser light source system configured to irradiate an edge of thesurface of the thermoelectric material layer with pulsed light tolocally heat the thermoelectric material layer, thereby generating aterahertz electromagnetic wave from the thermoelectric material layer.2. The terahertz electromagnetic wave generator of claim 1, wherein thepulsed laser light source system is configured to form a light spotwhich crosses the edge of the surface of the thermoelectric materiallayer.
 3. The terahertz electromagnetic wave generator of claim 1,wherein the terahertz electromagnetic wave has a frequency fallingwithin the range of 0.1 THz to 100 THz.
 4. The terahertz electromagneticwave generator of claim 3, wherein the pulsed laser light source systemcomprises: a light source which emits the pulsed light, of which thepulse width falls within the range of 1 femtosecond to 1 nanosecond; andan optical system which guides the pulsed light that is emitted from thelight source toward the edge of the surface of the thermoelectricmaterial layer.
 5. The terahertz electromagnetic wave generator of claim4, wherein the light source is a femtosecond laser light source.
 6. Theterahertz electromagnetic wave generator of claim 1, wherein thethermoelectric material layer is made of a material selected from thegroup consisting of a single-element thermoelectric material, analloy-based thermoelectric material, and an oxide-based thermoelectricmaterial.
 7. The terahertz electromagnetic wave generator of claim 6,wherein the thermoelectric material layer is made of at least onematerial selected from the group consisting of Bi, Sb, a BiTe-basedalloy, a PbTe-based alloy, an SiGe-based alloy, Ca_(x)CoO₂, Na_(x)CoO₂,and SrTiO₃.
 8. The terahertz electromagnetic wave generator of claim 1,wherein the thermoelectric material layer has a thickness of 10 nm to 1μm.
 9. The terahertz electromagnetic wave generator of claim 1, whereinthe substrate is configured to transmit the terahertz electromagneticwave.
 10. A terahertz spectrometer comprising: the terahertzelectromagnetic wave generator of claim 1; an optical system whichirradiates an object with a terahertz electromagnetic wave that isgenerated by the terahertz electromagnetic wave generator; and adetector which detects the terahertz electromagnetic wave that istransmitted through, or reflected from, the object.
 11. The terahertzspectrometer of claim 10, further comprising a processing apparatuswhich generates an image representing a terahertz electromagnetic wavewith a particular wavelength based on the output of the detector.
 12. Amethod of generating a terahertz electromagnetic wave, the methodcomprising: (A) providing a thermoelectric material body; and (B)locally heating the thermoelectric material body by irradiating aportion of the thermoelectric material body with pulsed light, the step(B) comprising: locally heating the thermoelectric material body so thatan asymmetric heat distribution is formed in the thermoelectric materialbody; and producing thermal diffusion current in the portion of thethermoelectric material body that is heated locally, thereby generatinga terahertz electromagnetic wave.